Biodiesel and biodiesel blend fuels

ABSTRACT

Fuel compositions are provided which comprise synergistic combinations of a biodiesel or biodiesel blend, an antioxidant comprising a mixture of at least one diphenol and at least one sterically hindered phenol, and a cetane improver. The fuel compositions described herein exhibit a unique combination of improved oxidation stability and enhanced ignition quality.

This application claims the benefit of U.S. Provisional Application 61/273,181 filed Jul. 31, 2009.

FIELD OF THE INVENTION

The invention provides fuel compositions comprising synergistic combinations of a biodiesel or biodiesel blend, an antioxidant comprising a mixture of at least one diphenol and at least one sterically hindered phenol, and a cetane improver.

BACKGROUND OF THE INVENTION

Biodiesel is the name for a variety of ester-based oxygenated fuels made from vegetable oils, fats, greases, or other sources of triglycerides. Biodiesel is a nontoxic and biodegradable blendstock which may be blended with petroleum diesel provided relevant specifications are met. Blends of biodiesel with petroleum diesel can substantially reduce the emission levels and toxicity of diesel exhaust. Biodiesel has been designated as an alternative fuel by the United States Department of Energy and the United States Department of Transportation, and is registered with the United States Environmental Protection Agency as a fuel and fuel additive.

Biodiesel fuels typically contain unsaturated fatty acid esters that can easily oxidize in the presence of oxygen, especially if UV light, and/or trace metals are also present. The products formed from this oxidation give rise to sediment and gum formation within the fuel that can lead to corrosion and plugging of filters, pumps and fuel injectors which utilize fuel containing biodiesel.

While antioxidants such as sterically-hindered phenols have been used in an effort to stabilize biodiesels, there remains a need for a biodiesel composition that has both improved oxidation stability and ignition quality that reduces or eliminates gum formation within the fuel, and that reduces or eliminates corrosion and plugging of filters, vehicular injection pumps and fuel injectors.

SUMMARY OF THE INVENTION

We have discovered fuel compositions comprising synergistic combinations of a biodiesel (B 100) or biodiesel blend (Bx), an antioxidant comprising a mixture of at least one diphenol and at least one sterically hindered phenol, and a cetane improver. Notably, the fuel compositions described herein evidence enhanced oxidation stability as determined by the Rancimat test method (prEN 15751), even though they contain an ignition-enhancing, oxidation stability-degrading cetane improver.

In one aspect, the fuel compositions comprise: (1) between about 0.01% to about 1% by volume of a cetane improver; (2) either (a) a biodiesel, or (b) a biodiesel blend comprising about 2% to about 98% by volume of a biodiesel and about 2% to about 98% by volume of a petroleum distillate; and (3) an antioxidant comprising a mixture of at least one diphenol and at least one sterically hindered phenol, wherein (a) the amount of the antioxidant is (i) between about 20 milligrams per kilogram of the biodiesel or biodiesel blend to about 10,000 milligrams per kilogram of the biodiesel or biodiesel blend, or (ii) is between about 16 milligrams per liter of the biodiesel or biodiesel blend to about 8,000 milligrams per liter of the biodiesel or biodiesel blend, and (b) wherein the volumetric ratio of the diphenol(s) to the sterically hindered phenol(s) is between about 0.01:1 to about 10:1.

Given their unique combination of improved oxidation stability, enhanced ignition quality and the fact that an antioxidant combination of a sterically hindered phenol and diphenol is cheaper than a pure diphenol antioxidant, the fuel compositions described herein facilitate a wider use of biodiesels and biodiesel blends. Further, the fuel compositions described herein should avoid the corrosion, plugging, and sediment and gum formation associated with known biodiesels and biodiesel blends.

These and other aspects of the invention are described further in the following detailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 and FIG. 2 illustrate that the addition of a cetane improver to biodiesel fuels reduced their oxidation stability measured by the Rancimat test method (prEN 15751), as determined in the experiment of Example 1. As described in Examples 1-4 hereinafter, the fuels tested were Fuel 1 (5% by volume of soybean methyl ester plus 95% by volume of petroleum diesel fuel), Fuel 2 (99.7% by volume of Fuel 1+0.3% by volume of 2-ethylhexyl nitrate), and variants of Fuel 2.

FIG. 3 illustrates the oxidation-stabilizing effect of adding a diphenol and a sterically hindered phenol to biodiesel fuels containing a cetane improver, as determined in the experiment of Example 2.

FIG. 4 illustrates that the presence of a diphenol and a sterically hindered phenol did not significantly affect the ignition quality of biodiesel fuels containing a cetane improver, as determined in the experiment of Example 3.

FIG. 5 illustrates that a benzoquinone proved ineffective as an oxidation-stabilizing substitute for a diphenol, as determined in the experiment of Example 4.

DETAILED DESCRIPTION OF THE INVENTION

Unless otherwise stated, all percentages disclosed herein are on a volume basis.

Any end point of a range stated herein can be combined with any other end point to form another suitable range.

The following definitions apply unless indicated otherwise.

“Biodiesel” means a composition that can be used as a fuel for diesel engines and that contains at least about 50% by weight of esters of saturated and unsaturated fatty acids, including fatty acid methyl esters (FAME's), fatty acid ethyl esters (FAEE's), propyl esters of fatty acids, or combinations of two or more methyl, ethyl, and propyl esters. In one example, a biodiesel comprises between about 50% to about 99% by weight of methyl esters of saturated and unsaturated fatty acids, where the methyl esters of saturated and unsaturated fatty acids include C₈-C₂₄ fatty acid methyl esters, where C₅-C₂₄ indicates the number of carbons in the original fatty acid.

Biodiesels can be made by transesterification of one or more vegetable oils, animal fats, or mixtures thereof (e.g. soybean oil, rapeseed oil, palm oil, canola oil, sunflower oil, olive oil, corn oil, tallow oil, coconut oil, jatropha oil, yellow grease, animal fats, used cooking oil, and mixtures thereof) with an alcohol such as methanol or ethanol. In one aspect the fatty acid esters are largely unsaturated and comprise a rapeseed methyl ester, a canola methyl ester, a soybean methyl ester, a corn oil methyl ester, or a mixture thereof. A 100% biodiesel (B100) should meet ASTM D6751 and/or EN 14214 specifications.

Biodiesel blends (biodiesel blended with a petroleum distillate such as diesel fuel) (Bx) have a composition reflective of blend ratio and the distillate chosen for the blend.

“Petroleum distillate” includes naphtha or middle distillates including kerosene and diesel.

A non-limiting example of a “diesel fuel” or “a diesel” is composed of a mixture of C₉-C₂₄ hydrocarbons that comprise about 50% to about 95% by volume of aliphatic hydrocarbons, of which about 0% to about 50% by volume are cycloparaffins, about 0% to about 5% by volume of olefinic hydrocarbons, and about 5% to about 50% by volume of aromatic hydrocarbons, and which boil at between about 280° F. (138° C.) and 750° F. (399° C.).

A non-limiting example of a “kerosene” comprises about 5% to about 50% by volume of an aromatic fraction, about 0% to about 50% by volume of a cycloparaffin fraction, and about 0% to about 5% by volume of an olefinic fraction, with the rest comprising aliphatic hydrocarbons.

An aromatics fraction can contain methyl aromatics and non-methyl alkyl aromatics. Non-limiting examples of non-methyl alkyl aromatics include molecules such as, alkyl benzenes, dialkylbenzenes, alkylnaphthalenes, alkyl biphenyls, and alkyl phenanthrenes, and the like, in which one or more linear or branched alkyl groups containing two or more carbons is bonded to the aromatic ring. Non-limiting examples of methyl aromatics include aromatic molecules such as methylnaphthalene, dimethylnaphthalenes, and the like.

A cycloparaffin fraction consists of cycloalkanes or molecules containing at least one cycloalkane ring. Non-limiting examples of components of the cycloparaffin fraction include alkylcyclohexanes and alkylcyclopentanes.

An olefinic fraction can contain linear, branched, and cyclo-olefins. Non-limiting examples of components of the olefinic fraction include dodecenes and hexadecenes.

A “cetane improver” includes but is not limited to 2-ethylhexyl nitrate (EHN) (e.g. HITEC® 4103, Ethyl Corp., Richmond, Va.), cyclohexyl nitrate, di-tert-butyl peroxide, methyl nitrate, ethyl nitrate, n-propyl nitrate, isopropyl nitrate, allyl nitrate, n-butyl nitrate, isobutyl nitrate, sec-butyl nitrate, tert-butyl nitrate, n-amyl nitrate, isoamyl nitrate, 2-amyl nitrate, 3-amyl nitrate, tert-amyl nitrate, n-hexyl nitrate, 2-ethylhexyl nitrate, n-heptyl nitrate, sec-heptyl nitrate, n-octyl nitrate, sec-octyl nitrate, n-nonyl nitrate, n-decyl nitrate, n-dodecyl nitrate, cyclopentylnitrate, cyclohexylnitrate, methylcyclohexyl nitrate, isopropylcyclohexyl nitrate, and the esters of alkoxy substituted aliphatic alcohols, such as 1-methoxypropyl-2-nitrate, 1-ethoxpropyl-2 nitrate, 1-isopropoxy-butyl nitrate, 1-ethoxylbutyl nitrate, and mixtures thereof.

A “diphenol” includes but is not limited to hydroquinone (HQ), mono and dialkylated hydroquinones having one or two C₁-C₈ alkyl groups (e.g. tert-butylhydroquinone (TBHQ), 2-tert-butyl-1,4-hydroquinone, 2,5-di-tert-butyl-1,4-hydroquinone, methylhydroquinone (toluhydroquinone or THQ), 2,5-di-tert.-octylhydroquinone (DOH)), and mixtures thereof.

A “sterically hindered phenol” includes but is not limited to mono, di, and trialkylated phenols such as 2,6-di-tert-butylphenol, 2,4-di-tert-butylphenol, 2,6-di-tert-butyl-4-methylphenol (BHT), 2-tert-butylphenol, 2,4,6-tri-tert-butylphenol, 2,6-di-tert-butyl-4-dimethylaminomethylphenol, and mixtures thereof. Commercially-available sterically hindered phenols that can be used in the fuel compositions described herein include Ethanox® 4703 and 4733 (Albermarle Corp., Baton Rouge, La.), Westco® AO-29 (Western Reserve Chemical, Stow, Ohio), Baynox® (Lanxess, Leverkusen, Del.), and Ionol® BF 150 and BF 200 (Oxiris Chemicals S.A., Barcelona, ES).

In certain aspects, the diphenol and sterically hindered phenol are selected, respectively, from a compound of formula (I) and a compound of formula (II):

wherein R₁ and R₃ are H or a tert-butyl group and R₂ is H, CH₃, a tert-butyl group or —CH₂—N(CH₃)₂.

In one aspect, fuel compositions of the invention may also include an aromatic amine antioxidant (e.g. a phenylediamine-type antioxidant) such as N, N′-di-sec-butyl-p-phenylenediamine, 4-isopropylaminodiphenylamine, phenyl-naphthyl amine, and ring-alkylated diphenylamines.

Fuel compositions of the invention may also include performance additives such as cold flow additives, cloud point depressants, biocides, conductivity improvers, corrosion inhibitors, metal deactivators, and engine cleaning agents. In some aspects, such additives are present in an amount which ranges from about 0.001 to about 2.0% by weight of the fuel composition.

In certain aspects, the fuel compositions comprise: (1) between about 0.01% to about 1%, in some aspects between about 0.01% to about 0.75%, in some aspects between 0.01% to about 0.5%, and in some aspects between about 0.01% to about 0.35% by volume of a cetane improver; (2) either (a) a biodiesel comprising at least about 50% by weight, in some aspects at least about 60% by weight, in some aspects at least about 70% by weight, in some aspects at least about 80% by weight, in some aspects at least about 90% by weight, in some aspects at least about 95% by weight of esters of saturated and unsaturated fatty acids, or (b) a biodiesel blend comprising about 2% to about 98% by volume of a biodiesel as described herein and about 2% to about 98% by volume of a petroleum distillate, or in some aspects about 10% to about 90% by volume of a biodiesel and about 10% to about 90% by volume of a petroleum distillate, or in some aspects about 20% to about 80% by volume of a biodiesel and about 20% to about 80% by volume of a petroleum distillate, or in some aspects about 30% to about 70% by volume of a biodiesel and about 30% to about 70% by volume of a petroleum distillate, or in some aspects about 40% to about 60% by volume of a biodiesel and about 40% to about 60% by volume of a petroleum distillate, or in some aspects about 50% by volume of a biodiesel and about 50% by volume of a petroleum distillate, or in some aspects about 30% by volume of a biodiesel and about 70% by volume of a petroleum distillate, or in some aspects about 20% by volume of a biodiesel and about 80% by volume of a petroleum distillate, or in some aspects about 5-10% by volume of a biodiesel and about 90-95% by volume of a petroleum distillate; and (3) an antioxidant comprising a mixture of at least one diphenol and at least one sterically hindered phenol, wherein (a) the amount of the antioxidant is (i) between about 20 milligrams per kilogram of the biodiesel or biodiesel blend to about 10,000 milligrams per kilogram of the biodiesel or biodiesel blend, or in some aspects is between about 100 milligrams per kilogram of the biodiesel or biodiesel blend to about 5,000 milligrams per kilogram of the biodiesel or biodiesel blend, or in some aspects is between about 100 milligrams per kilogram of the biodiesel or biodiesel blend to about 1,000 milligrams per kilogram of the biodiesel or biodiesel blend, or in some aspects is between about 100 milligrams per kilogram of the biodiesel or biodiesel blend to about 500 milligrams per kilogram of the biodiesel or biodiesel blend, or (ii) in some aspects is between about 16 milligrams per liter of the biodiesel or biodiesel blend to about 8,000 milligrams per liter of the biodiesel or biodiesel blend, or in some aspects is between about 100 or about 200 milligrams per liter of the biodiesel or biodiesel blend to about 400 or about 500 milligrams per liter of the biodiesel or biodiesel blend, and wherein (b) the volumetric ratio of the diphenol(s) to the sterically hindered phenol(s) is between about 0.01:1 to about 10:1, or in some aspects is about 0.1:1, or in some aspects is about 0.25:1, or in some aspects is about 0.5:1, or in some aspects is about 0.75:1, or in some aspects is about 0.9:1, or in some aspects is about 1:1, or in some aspects is about 1.25:1, or in some aspects is about 1.5:1.

In some aspects, the fuel compositions comprise: (1) between about 0.1% to about 0.35% by volume of a cetane improver; (2) a biodiesel blend comprising (a) about 5% to about 30% by volume of a biodiesel comprising a fatty acid ester selected from the group consisting of rapeseed methyl ester, a canola methyl ester, a soybean methyl ester, a corn oil methyl ester, or a mixture thereof, and (b) about 70 to about 95% by volume of a diesel; and (3) an antioxidant comprising (a) a diphenol selected from the group consisting of tert-butylhydroquinone, 2-tert-butyl-1,4-hydroquinone, 2,5-di-tert-butyl-1,4-hydroquinone, methylhydroquinone, and 2,5-di-tert.-octylhydroquinone, and (b) a sterically hindered phenol selected from the group consisting of 2,6-di-tert-butylphenol, 2,4-di-tert-butylphenol, 2,6-di-tert-butyl-4-methylphenol, 2-tert-butylphenol, 2,4,6-tri-tert-butylphenol, and 2,6-di-tert-butyl-4-dimethylaminomethylphenol, wherein (a) the amount of the antioxidant is between about 100 milligrams per liter biodiesel blend to about 500 milligrams per liter of the biodiesel blend, and wherein (b) the volumetric ratio of the diphenol to the sterically hindered phenol is between about 0.5:1 to about 1.5:1.

In some aspects, the fuel compositions comprise: (1) between about 0.1% to about 0.35% by volume of a cetane improver; (2) a biodiesel blend comprising (a) about 5% to about 15% by volume of a biodiesel comprising a fatty acid ester selected from the group consisting of rapeseed methyl ester, a canola methyl ester, a soybean methyl ester, a corn oil methyl ester, or a mixture thereof, and (b) about 85 to about 95% by volume of a diesel; and (3) an antioxidant comprising (a) a diphenol selected from the group consisting of tert-butylhydroquinone, 2-tert-butyl-1,4-hydroquinone, 2,5-di-tert-butyl-1,4-hydroquinone, methylhydroquinone, and 2,5-di-tert.-octylhydroquinone, and (b) a sterically hindered phenol selected from the group consisting of 2,6-di-tert-butylphenol, 2,4-di-tert-butylphenol, 2,6-di-tert-butyl-4-methylphenol, 2-tert-butylphenol, 2,4,6-tri-tert-butylphenol, and 2,6-di-tert-butyl-4-dimethylaminomethylphenol, wherein (a) the amount of the antioxidant is between about 200 milligrams per liter of the biodiesel blend to about 400 milligrams per liter of the biodiesel blend, and wherein (b) the volumetric ratio of the diphenol to the sterically hindered phenol is between about 0.75:1 to about 1:1.

In some aspects, the fuel compositions comprise: (1) between about 0.1% to about 0.35% by volume of a cetane improver; (2) a biodiesel blend comprising (a) about 5% to about 15% by volume of a biodiesel comprising between about 50% to about 99% by weight of methyl, ethyl, and propyl esters of saturated and unsaturated fatty acids, and (b) about 85 to about 95% by volume of a diesel; and (3) an antioxidant comprising (a) a diphenol selected from the group consisting of tert-butylhydroquinone, 2-tert-butyl-1,4-hydroquinone, 2,5-di-tert-butyl-1,4-hydroquinone, methylhydroquinone, and 2,5-di-tert.-octylhydroquinone, and (b) a sterically hindered phenol selected from the group consisting of 2,6-di-tert-butylphenol, 2,4-di-tert-butylphenol, 2,6-di-tert-butyl-4-methylphenol, 2-tert-butylphenol, 2,4,6-tri-tert-butylphenol, and 2,6-di-tert-butyl-4-dimethylaminomethylphenol, wherein (a) the amount of the antioxidant is between about 200 milligrams per liter of the biodiesel blend to about 400 milligrams per liter of the biodiesel blend, and wherein (b) the volumetric ratio of the diphenol to the sterically hindered phenol is between about 0.75:1 to about 1:1.

In some aspects, the fuel compositions comprise: (1) between about 0.1% to about 0.35% by volume of 2-ethylhexyl nitrate; (2) a biodiesel blend comprising (a) about 5% to about 10% by volume of a biodiesel comprising a fatty acid ester selected from the group consisting of rapeseed methyl ester, a canola methyl ester, a soybean methyl ester, a corn oil methyl ester, or a mixture thereof, and (b) about 90% to about 95% by volume of a diesel; and (3) an antioxidant comprising (a) a diphenol selected from the group consisting of tert-butylhydroquinone, 2-tert-butyl-1,4-hydroquinone, 2,5-di-tert-butyl-1,4-hydroquinone, methylhydroquinone, and 2,5-di-tert.-octylhydroquinone, and (b) a sterically hindered phenol selected from the group consisting of 2,6-di-tert-butylphenol, 2,4-di-tert-butylphenol, 2,6-di-tert-butyl-4-methylphenol, 2-tert-butylphenol, 2,4,6-tri-tert-butylphenol, and 2,6-di-tert-butyl-4-dimethylaminomethylphenol, wherein (a) the amount of the antioxidant is between about 200 milligrams per liter of the biodiesel blend to about 400 milligrams per liter of the biodiesel blend, and wherein (b) the volumetric ratio of the diphenol to the sterically hindered phenol is between about 0.9:1 to about 1:1.

In some aspects, the fuel compositions comprise: (1) between about 0.1% to about 0.35% by volume of 2-ethylhexyl nitrate; (2) a biodiesel blend comprising (a) about 5% to about 10% by volume of a biodiesel comprising a fatty acid ester selected from the group consisting of rapeseed methyl ester, a canola methyl ester, a soybean methyl ester, a corn oil methyl ester, or a mixture thereof, and (b) about 90% to about 95% by volume of a diesel; and (3) a diphenol and a sterically hindered phenol selected, respectively, from a compound of formula (I) and a compound of formula (II):

wherein: (a) R₁ and R₃ are H or a tert-butyl group, and R₂ is H, CH₃, a tert-butyl group or —CH₂—N(CH₃)₂; (b) the amount of the antioxidant is between about 100 milligrams per liter biodiesel blend to about 500 milligrams per liter of the biodiesel blend, and wherein (c) the volumetric ratio of the diphenol to the sterically hindered phenol is between about 0.5:1 to about 1.5:1.

In some aspects, the fuel compositions comprise: (1) between about 0.1% to about 0.35% by volume of 2-ethylhexyl nitrate; (2) a biodiesel comprising about a fatty acid ester selected from the group consisting of rapeseed methyl ester, a canola methyl ester, a soybean methyl ester, a corn oil methyl ester, or a mixture thereof; and (3) a diphenol and a sterically hindered phenol selected, respectively, from a compound of formula (I) and a compound of formula (II):

wherein (a) R₁ and R₃ are H or a tert-butyl group, and R₂ is H, CH₃, a tert-butyl group or —CH₂—N(CH₃)₂; (b) the amount of the antioxidant is between about 200 milligrams per liter of the biodiesel blend to about 400 milligrams per liter of the biodiesel blend, and wherein (c) the volumetric ratio of the diphenol to the sterically hindered phenol is between about 0.9:1 to about 1:1.

These and other aspects of the fuel compositions described herein are illustrated further in the following examples, which are illustrative and are not limiting.

Experimental Apparatus and Methods

The following experimental apparatus and methods were used in the experiments of Examples 1-4 described below.

Experiments were undertaken in accord with the Rancimat test method (prEN 15751). The Rancimat test is carried out by passing a steady stream of air through a heated (110° C.) sample and measuring the volatile oxidation species produced over a period of time. The point at which the rate of production of the volatile oxidation species reaches a maximum is defined as the induction period or oxidation stability (measured in hours at the given temperature) and provides a characteristic value for oxidation stability.

Specifically, oxidation is induced by passing a stream of purified air at the rate of 10 liters/hr through the biodiesel sample (approx. 7.5 g), kept at constant temperature. The vapors released during the oxidation process, together with the air, are passed into the flask containing 60 ml of water which has been demineralized or distilled and contains an electrode for measuring the conductivity. The electrode is connected to a measuring and recording device. The end of the induction period is indicated when the conductivity begins to increase rapidly. This accelerated increase in conductivity is caused by the dissociation of volatile carboxylic acids which are produced during the oxidation process and which are absorbed in the water. When the conductivity of this measuring solution is recorded continuously, an oxidation curve is obtained whose point of inflection indicates the induction period; which provides the characteristic value for the oxidation stability. The EN 14214 Standard for the B100 and ASTM D7467 Standard for the B6-B20 biodiesel fuels specify a Rancimat induction period of six hours minimum.

Example 1

We unexpectedly found that the presence of a cetane improver such as 2-ethylhexyl nitrate improves the ignition quality of, and degrades the Rancimat oxidation stability of, a biodiesel blend or a biodiesel. Sterically hindered phenolic antioxidants improved the oxidation stability of the biodiesel or biodiesel blend fuels, but not to the same extent as a diphenol such as tert-butylhydroquinone antioxidant. However, a combination of a sterically hindered phenolic antioxidant with a diphenol such as tert-butylhydroquinone provided similar, and in some cases even directionally better, oxidation stability when compared to the use of only a diphenol.

The experiment of this example shows that the addition of a 2-ethylhexyl nitrate cetane improver to a biodiesel fuel reduced its Rancimat (prEN 15751) oxidation stability by about 26.5% (average of 2 Rancimat stability results). As listed below and as depicted in FIGS. 1 and 2, upon addition of the cetane improver, oxidation stability decreased for each of the fuels tested.

FIG. 1

Fuel 1: 5 vol % Soybean Methyl Ester+95 vol % petroleum diesel fuel Fuel 2: 99.7 vol % Fuel 1+0.3 vol % 2-ethylhexyl nitrate

Comparative Examples (FIG. 2)

B5 SME (no EHN) 28.4 hours B5 SME+0.1 vol % EHN 23.7 hours (16.6% Rancimat stability reduction) B5 SME+0.2 vol % EHN 20.4 hours (28.2% Rancimat stability reduction) B5 SME+0.35 vol % EHN 10.3 hours (63.7% Rancimat stability reduction)

Example 2

The experiment of this example shows that sterically hindered phenolic antioxidants improve the Rancimat stability of biodiesel fuel containing a cetane improver, but not to the same extent as a diphenol such as 2-tert-butylhydroquinone. However, as seen in FIG. 3, a mixture of a 50% by volume 2-tert-butylhydroquinone with 50% by volume of sterically hindered phenols provided significantly better Rancimat stability than the calculated Rancimat stability for that composition.

FIG. 3

Fuel 3: Fuel 2+400 mg/L Ethanox 4733 Fuel 4: Fuel 2+400 mg/L Ethanox 4703 Fuel 5: Fuel 2+400 mg/L 2-tert-butyl-1,4-hydroquinone Fuel 6: Fuel 2+200 mg/L Ethanox 4703+200 mg/L 2-tert-butyl-1,4-hydroquinone Fuel 7: Fuel 2+200 mg/L Ethanox 4733+200 mg/L 2-tert-butyl-1,4-hydroquinone Fuel 8: Fuel 6 calculated Rancimat stability Fuel 9: Fuel 7 calculated Rancimat stability

Example 3

The experiment of this example shows (as depicted in FIG. 4) that the 2-ethylhexyl nitrate cetane improver increases the ASTM D6890 ignition quality of the biodiesel Fuels 1-7 and that the presence of antioxidants did not significantly affect the ignition quality of the biodiesel fuels. Fuels 1-7 were comprised of Fuels 1 or 2 (Example 1).

FIG. 4

Fuel 1: 5 vol % Soybean Methyl Ester+95 vol % petroleum diesel fuel Fuel 2: 99.7 vol % Fuel 1+0.3 vol % 2-ethylhexyl nitrate Fuel 3: Fuel 2+400 mg/L Ethanox 4733 Fuel 4: Fuel 2+400 mg/L Ethanox 4703 Fuel 5: Fuel 2+400 mg/L 2-tert-butyl-1,4-hydroquinone Fuel 6: Fuel 2+200 mg/L Ethanox 4703+200 mg/L 2-tert-butyl-1,4-hydroquinone Fuel 7: Fuel 2+200 mg/L Ethanox 4733+200 mg/L 2-tert-butyl-1,4-hydroquinone

Example 4

The experiment of this example shows (as depicted in FIG. 5) that compared to a diphenol such as tert-butylhydroquinone, a benzoquinone such as 2,6-di-tert-butyl[1,4]benzoquinone (formula (III)) proves ineffective as an antioxidant.

FIG. 5

Fuel 2: 99.7 vol % Fuel 1+0.3 vol % 2-ethylhexyl nitrate Fuel 3: Fuel 2+400 mg/L Ethanox 4733 Fuel 4: Fuel 2+400 mg/L Ethanox 4703 Fuel 10: Fuel 2+400 mg/L 2,6-di-tert-butylbenzoquinone Fuel 11: Fuel 2+200 mg/L Ethanox 4733+200 mg/L 2,6-di-tert-butylbenzoquinone Fuel 12: Fuel 2+200 mg/L Ethanox 4703+200 mg/L 2,6-di-tert-butylbenzoquinone Fuels 2, 3, 4, 10, 11, and 12 were comprised of Fuels 1 or 2 (Example 1).

It is to be understood that the above description is intended for illustrative purposes only and is not intended to limit the scope of the present invention in any way. 

1. A fuel composition comprising: (a) between about 0.01% to about 1% by volume of a cetane improver; (b) a biodiesel blend comprising about 2% to about 98% by volume of a biodiesel and about 2% to about 98% by volume of a petroleum distillate; and (c) an antioxidant comprising a mixture of a diphenol and a sterically hindered phenol, wherein (1) the amount of the antioxidant is (i) between about 20 milligrams per kilogram to about 10,000 milligrams per kilogram of the biodiesel blend, or (ii) is between about 16 milligrams per liter to about 8,000 milligrams per liter of the biodiesel blend, and (2) wherein the volumetric ratio of the diphenol to the sterically hindered phenol is between about 0.01:1 to about 10:1.
 2. The fuel composition of claim 1, wherein: (a) the cetane improver is selected from the group consisting of 2-ethylhexyl nitrate, cyclohexyl nitrate, di-tert-butyl peroxide, methyl nitrate, ethyl nitrate, n-propyl nitrate, isopropyl nitrate, allyl nitrate, n-butyl nitrate, isobutyl nitrate, sec-butyl nitrate, tert-butyl nitrate, n-amyl nitrate, isoamyl nitrate, 2-amyl nitrate, 3-amyl nitrate, tert-amyl nitrate, n-hexyl nitrate, 2-ethylhexyl nitrate, n-heptyl nitrate, sec-heptyl nitrate, n-octyl nitrate, sec-octyl nitrate, n-nonyl nitrate, n-decyl nitrate, n-dodecyl nitrate, cyclopentylnitrate, cyclohexylnitrate, methylcyclohexyl nitrate, isopropylcyclohexyl nitrate, 1-methoxypropyl-2-nitrate, 1-ethoxpropyl-2 nitrate, 1-isopropoxy-butyl nitrate, 1-ethoxylbutyl nitrate, and mixtures thereof; (b) the biodiesel blend comprises (1) between about 2% to about 98% by volume of a biodiesel comprising between about 50% to about 99% by weight of methyl, ethyl, and propyl esters of saturated and unsaturated fatty acids, and (2) between about 2% to about 98% by volume of a petroleum distillate; (c) the diphenol is selected from the group consisting of hydroquinone, tert-butylhydroquinone, 2-tert-butyl-1,4-hydroquinone, 2,5-di-tert-butyl-1,4-hydroquinone, methylhydroquinone, 2,5-di-tert.-octylhydroquinone, and mixtures thereof; and (d) the sterically hindered phenol is selected from the group consisting of 2,6-di-tert-butylphenol, 2,4-di-tert-butylphenol, 2,6-di-tert-butyl-4-methylphenol, 2-tert-butylphenol, 2,4,6-tri-tert-butylphenol, 2,6-di-tert-butyl-4-dimethylaminomethylphenol, and mixtures thereof.
 3. The fuel composition of claim 1, wherein: (a) the cetane improver is selected from the group consisting of 2-ethylhexyl nitrate, cyclohexyl nitrate, di-tert-butyl peroxide, methyl nitrate, ethyl nitrate, n-propyl nitrate, isopropyl nitrate, allyl nitrate, n-butyl nitrate, isobutyl nitrate, sec-butyl nitrate, tert-butyl nitrate, n-amyl nitrate, isoamyl nitrate, 2-amyl nitrate, 3-amyl nitrate, tert-amyl nitrate, n-hexyl nitrate, 2-ethylhexyl nitrate, n-heptyl nitrate, sec-heptyl nitrate, n-octyl nitrate, sec-octyl nitrate, n-nonyl nitrate, n-decyl nitrate, n-dodecyl nitrate, cyclopentylnitrate, cyclohexylnitrate, methylcyclohexyl nitrate, isopropylcyclohexyl nitrate, 1-methoxypropyl-2-nitrate, 1-ethoxpropyl-2 nitrate, 1-isopropoxy-butyl nitrate, 1-ethoxylbutyl nitrate and mixtures thereof; (b) the biodiesel blend comprises (1) between about 2% to about 98% by volume of a biodiesel formed by transesterifying an alcohol with a composition selected from the group consisting of soybean oil, rapeseed oil, palm oil, canola oil, sunflower oil, olive oil, corn oil, coconut oil, jatropha oil, tallow oil, yellow grease animal fats, used cooking oil, and mixtures thereof, and (2) between about 2% to about 98% of a petroleum distillate; (c) the diphenol is selected from the group consisting of hydroquinone, tert-butylhydroquinone, 2-tert-butyl-1,4-hydroquinone, 2,5-di-tert-butyl-1,4-hydroquinone, methylhydroquinone, 2,5-di-tert.-octylhydroquinone and mixtures thereof; and (d) the sterically hindered phenol is selected from the group consisting of 2,6-di-tert-butylphenol, 2,4-di-tert-butylphenol, 2,6-di-tert-butyl-4-methylphenol, 2-tert-butylphenol, 2,4,6-tri-tert-butylphenol, 2,6-di-tert-butyl-4-dimethylaminomethylphenol, and mixtures thereof.
 4. The fuel composition of claim 1, wherein: (a) the cetane improver is selected from the group consisting of 2-ethylhexyl nitrate, cyclohexyl nitrate, di-tert-butyl peroxide, methyl nitrate, ethyl nitrate, n-propyl nitrate, isopropyl nitrate, allyl nitrate, n-butyl nitrate, isobutyl nitrate, sec-butyl nitrate, tert-butyl nitrate, n-amyl nitrate, isoamyl nitrate, 2-amyl nitrate, 3-amyl nitrate, tert-amyl nitrate, n-hexyl nitrate, 2-ethylhexyl nitrate, n-heptyl nitrate, sec-heptyl nitrate, n-octyl nitrate, sec-octyl nitrate, n-nonyl nitrate, n-decyl nitrate, n-dodecyl nitrate, cyclopentylnitrate, cyclohexylnitrate, methylcyclohexyl nitrate, isopropylcyclohexyl nitrate, 1-methoxypropyl-2-nitrate, 1-ethoxpropyl-2 nitrate, 1-isopropoxy-butyl nitrate, 1-ethoxylbutyl nitrate and mixtures thereof; (b) the biodiesel blend comprises (1) between about 2% to about 98% by volume of a biodiesel comprising a fatty acid ester selected from the group consisting of rapeseed methyl ester, a canola methyl ester, a soybean methyl ester, a corn oil methyl ester, or a mixture thereof, and (2) between about 2% to about 98% by volume of a diesel; (c) the diphenol is selected from the group consisting of hydroquinone, tert-butylhydroquinone, 2-tert-butyl-1,4-hydroquinone, 2,5-di-tert-butyl-1,4-hydroquinone, methylhydroquinone, 2,5-di-tert.-octylhydroquinone, and mixtures thereof; and (d) the sterically hindered phenol is selected from the group consisting of 2,6-di-tert-butylphenol, 2,4-di-tert-butylphenol, 2,6-di-tert-butyl-4-methylphenol, 2-tert-butylphenol, 2,4,6-tri-tert-butylphenol, 2,6-di-tert-butyl-4-dimethylaminomethylphenol, and mixtures thereof.
 5. The fuel composition of claim 1, wherein: (a) the cetane improver is selected from the group consisting of 2-ethylhexyl nitrate, cyclohexyl nitrate, di-tert-butyl peroxide, methyl nitrate, ethyl nitrate, n-propyl nitrate, isopropyl nitrate, allyl nitrate, n-butyl nitrate, isobutyl nitrate, sec-butyl nitrate, tert-butyl nitrate, n-amyl nitrate, isoamyl nitrate, 2-amyl nitrate, 3-amyl nitrate, tert-amyl nitrate, n-hexyl nitrate, 2-ethylhexyl nitrate, n-heptyl nitrate, sec-heptyl nitrate, n-octyl nitrate, sec-octyl nitrate, n-nonyl nitrate, n-decyl nitrate, n-dodecyl nitrate, cyclopentylnitrate, cyclohexylnitrate, methylcyclohexyl nitrate, isopropylcyclohexyl nitrate, 1-methoxypropyl-2-nitrate, 1-ethoxpropyl-2 nitrate, 1-isopropoxy-butyl nitrate, 1-ethoxylbutyl nitrate and mixtures thereof; (b) the biodiesel blend comprises (1) between about 2% to about 98% by volume of a biodiesel comprising a fatty acid ester selected from the group consisting of rapeseed methyl ester, a canola methyl ester, a soybean methyl ester, a corn oil methyl ester, or a mixture thereof, and (2) between about 2% to about 98% by volume of a diesel; and (c) the diphenol and sterically hindered phenol are selected, respectively, from a compound of formula (I) and a compound of formula (II):

wherein R₁ and R₃ are H or a tert-butyl group, and R₂ is H, CH₃, a tert-butyl group or —CH₂—N(CH₃)₂.
 6. The fuel composition of claim 1, wherein: (a) the fuel composition contains about 0.1% to about 0.35% by volume of a cetane improver; (b) the biodiesel blend comprises (1) about 5% to about 30% by volume of a biodiesel comprising a fatty acid ester selected from the group consisting of rapeseed methyl ester, a canola methyl ester, a soybean methyl ester, a corn oil methyl ester, or a mixture thereof, and (2) about 70 to about 90% by volume of a diesel; (c) the diphenol is selected from the group consisting of tert-butylhydroquinone, 2-tert-butyl-1,4-hydroquinone, 2,5-di-tert-butyl-1,4-hydroquinone, methylhydroquinone, and 2,5-di-tert.-octylhydroquinone, and mixtures thereof; (d) the sterically hindered phenol is selected from the group consisting of 2,6-di-tert-butylphenol, 2,4-di-tert-butylphenol, 2,6-di-tert-butyl-4-methylphenol, 2-tert-butylphenol, 2,4,6-tri-tert-butylphenol, and 2,6-di-tert-butyl-4-dimethylaminomethylphenol, and mixtures thereof; (e) the fuel composition contains about 100 milligrams of antioxidant per liter of the biodiesel blend to about 500 milligrams of antioxidant per liter of the biodiesel blend; and (f) the volumetric ratio of the diphenol to the sterically hindered phenol is between about 0.5:1 to about 1.5:1.
 7. The fuel composition of claim 1, wherein: (a) the fuel composition contains about 0.1% to about 0.35% by volume of a cetane improver; (b) the biodiesel blend comprises (1) about 5% to about 30% by volume of a biodiesel comprising a fatty acid ester selected from the group consisting of rapeseed methyl ester, a canola methyl ester, a soybean methyl ester, a corn oil methyl ester, or a mixture thereof, and (2) about 70 to about 95% by volume of a diesel; and (c) the diphenol is selected from the group consisting of tert-butylhydroquinone, 2-tert-butyl-1,4-hydroquinone, 2,5-di-tert-butyl-1,4-hydroquinone, methylhydroquinone, and 2,5-di-tert.-octylhydroquinone, and mixtures thereof; (d) the sterically hindered phenol is selected from the group consisting of 2,6-di-tert-butylphenol, 2,4-di-tert-butylphenol, 2,6-di-tert-butyl-4-methylphenol, 2-tert-butylphenol, 2,4,6-tri-tert-butylphenol, and 2,6-di-tert-butyl-4-dimethylaminomethylphenol, and mixtures thereof; (e) the fuel composition contains about 200 milligrams of antioxidant per liter of the biodiesel blend to about 400 milligrams of antioxidant per liter of the biodiesel blend; and (f) the volumetric ratio of the diphenol to the sterically hindered phenol is between about 0.75:1 to about 1:1.
 8. The fuel composition of claim 1, wherein: (a) the fuel composition contains about 0.1% to about 0.35% by volume of a cetane improver; (b) the biodiesel blend comprises (1) about 5% to about 15% by volume of a biodiesel comprising between about 50% to about 90% by weight of methyl, ethyl, and propyl esters of saturated and unsaturated fatty acids, and (2) about 85 to about 95% by volume of a diesel; (c) the diphenol is selected from the group consisting of tert-butylhydroquinone, 2-tert-butyl-1,4-hydroquinone, 2,5-di-tert-butyl-1,4-hydroquinone, methylhydroquinone, and 2,5-di-tert.-octylhydroquinone, and mixtures thereof; (d) the sterically hindered phenol is selected from the group consisting of 2,6-di-tert-butylphenol, 2,4-di-tert-butylphenol, 2,6-di-tert-butyl-4-methylphenol, 2-tert-butylphenol, 2,4,6-tri-tert-butylphenol, and 2,6-di-tert-butyl-4-dimethylaminomethylphenol, and mixtures thereof; (e) the fuel composition contains about 200 milligrams of antioxidant per liter of the biodiesel blend to about 400 milligrams of antioxidant per liter of the biodiesel blend; and
 9. The fuel composition of claim 1, wherein the fuel composition further comprises a composition selected from the group consisting of an aromatic amine antioxidant and a performance additive.
 10. The fuel composition of claim 1, wherein the fuel composition comprises: (a) about 0.3% by volume of 2-ethylhexyl nitrate; (b) a biodiesel blend comprising about 5% by volume of a biodiesel and about 95% by volume of a diesel; and (c) about 400 milligrams per liter of the biodiesel blend of an antioxidant comprising a mixture of 2-tert-butyl-1,4-hydroquinone and a sterically hindered phenol, wherein the volumetric ratio of 2-tert-butyl-1,4-hydroquinone to the sterically hindered phenol is about 1:1.
 11. The fuel composition of claim 1, wherein the fuel composition comprises: (a) about 0.3% by volume of 2-ethylhexyl nitrate; (b) a biodiesel blend comprising about 5% by volume of a biodiesel and about 95% by volume of a diesel; and (c) about 400 milligrams per liter of the biodiesel blend of an antioxidant comprising a mixture of 2-tert-butyl-1,4-hydroquinone and a sterically hindered phenol selected from the group consisting of Ethanox® 4703 and Ethanox® 4733, wherein the volumetric ratio of 2-tert-butyl-1,4-hydroquinone to the sterically hindered phenol is about 1:1.
 12. The fuel composition of claim 11, wherein the biodiesel comprises soybean methyl ester.
 13. A fuel composition comprising: (a) between about 0.01% to about 1% by volume of a cetane improver; (b) a biodiesel comprising between about 50% to about 99% by weight of methyl, ethyl, and propyl esters of saturated and unsaturated fatty acids; and (c) an antioxidant comprising a mixture of a diphenol and a sterically hindered phenol, wherein (1) the amount of the antioxidant is (i) between about 20 milligrams per kilogram to about 10,000 milligrams per kilogram of the biodiesel, or (ii) is between about 16 milligrams per liter to about 8,000 milligrams per liter of the biodiesel, and (2) wherein the volumetric ratio of the diphenol to the sterically hindered phenol is between about 0.01:1 to about 10:1.
 14. The fuel composition of claim 13, wherein: (a) the cetane improver is selected from the group consisting of 2-ethylhexyl nitrate, cyclohexyl nitrate, di-tert-butyl peroxide, methyl nitrate, ethyl nitrate, n-propyl nitrate, isopropyl nitrate, allyl nitrate, n-butyl nitrate, isobutyl nitrate, sec-butyl nitrate, tert-butyl nitrate, n-amyl nitrate, isoamyl nitrate, 2-amyl nitrate, 3-amyl nitrate, tert-amyl nitrate, n-hexyl nitrate, 2-ethylhexyl nitrate, n-heptyl nitrate, sec-heptyl nitrate, n-octyl nitrate, sec-octyl nitrate, n-nonyl nitrate, n-decyl nitrate, n-dodecyl nitrate, cyclopentylnitrate, cyclohexylnitrate, methylcyclohexyl nitrate, isopropylcyclohexyl nitrate, 1-methoxypropyl-2-nitrate, 1-ethoxpropyl-2 nitrate, 1-isopropoxy-butyl nitrate, 1-ethoxylbutyl nitrate and mixtures thereof; (b) the biodiesel is formed by transesterifying an alcohol with a composition selected from the group consisting of soybean oil, rapeseed oil, palm oil, canola oil, sunflower oil, olive oil, corn oil, tallow oil, coconut oil, jatropha oil, yellow grease animal fats, used cooking oil, and mixtures thereof; (c) the diphenol is selected from the group consisting of hydroquinone, tert-butylhydroquinone, 2-tert-butyl-1,4-hydroquinone, 2,5-di-tert-butyl-1,4-hydroquinone, methylhydroquinone, 2,5-di-tert.-octylhydroquinone, and mixtures thereof; and (d) the sterically hindered phenol is selected from the group consisting of 2,6-di-tert-butylphenol, 2,4-di-tert-butylphenol, 2,6-di-tert-butyl-4-methylphenol, 2-tert-butylphenol, 2,4,6-tri-tert-butylphenol, 2,6-di-tert-butyl-4-dimethylaminomethylphenol, and mixtures thereof.
 15. The fuel composition of claim 13, wherein: (a) the cetane improver is selected from the group consisting of 2-ethylhexyl nitrate, cyclohexyl nitrate, di-tent-butyl peroxide, methyl nitrate, ethyl nitrate, n-propyl nitrate, isopropyl nitrate, allyl nitrate, n-butyl nitrate, isobutyl nitrate, sec-butyl nitrate, tert-butyl nitrate, n-amyl nitrate, isoamyl nitrate, 2-amyl nitrate, 3-amyl nitrate, tert-amyl nitrate, n-hexyl nitrate, 2-ethylhexyl nitrate, n-heptyl nitrate, sec-heptyl nitrate, n-octyl nitrate, sec-octyl nitrate, n-nonyl nitrate, n-decyl nitrate, n-dodecyl nitrate, cyclopentylnitrate, cyclohexylnitrate, methylcyclohexyl nitrate, isopropylcyclohexyl nitrate, 1-methoxypropyl-2-nitrate, 1-ethoxpropyl-2 nitrate, 1-isopropoxy-butyl nitrate, 1-ethoxylbutyl nitrate and mixtures thereof; (b) the biodiesel comprises a fatty acid ester selected from the group consisting of rapeseed methyl ester, a canola methyl ester, a soybean methyl ester, a corn oil methyl ester, and mixtures thereof; (c) the diphenol is selected from the group consisting of hydroquinone, tert-butylhydroquinone, 2-tert-butyl-1,4-hydroquinone, 2,5-di-tert-butyl-1,4-hydroquinone, methylhydroquinone, 2,5-di-tert.-octylhydroquinone, and mixtures thereof; and (d) the sterically hindered phenol is selected from the group consisting of 2,6-di-tert-butylphenol, 2,4-di-tert-butylphenol, 2,6-di-tert-butyl-4-methylphenol, 2-tert-butylphenol, 2,4,6-tri-tert-butylphenol, 2,6-di-tert-butyl-4-dimethylaminomethylphenol, and mixtures thereof.
 16. the fuel composition of claim 13, wherein: (a) the cetane improver is selected from the group consisting of 2-ethylhexyl nitrate, cyclohexyl nitrate, di-tert-butyl peroxide, methyl nitrate, ethyl nitrate, n-propyl nitrate, isopropyl nitrate, allyl nitrate, n-butyl nitrate, isobutyl nitrate, sec-butyl nitrate, tert-butyl nitrate, n-amyl nitrate, isoamyl nitrate, 2-amyl nitrate, 3-amyl nitrate, tert-amyl nitrate, n-hexyl nitrate, 2-ethylhexyl nitrate, n-heptyl nitrate, sec-heptyl nitrate, n-octyl nitrate, sec-octyl nitrate, n-nonyl nitrate, n-decyl nitrate, n-dodecyl nitrate, cyclopentylnitrate, cyclohexylnitrate, methylcyclohexyl nitrate, isopropylcyclohexyl nitrate, 1-methoxypropyl-2-nitrate, 1-ethoxpropyl-2 nitrate, 1-isopropoxy-butyl nitrate, 1-ethoxylbutyl nitrate and mixtures thereof; (b) the biodiesel comprises a fatty acid ester selected from the group consisting of rapeseed methyl ester, a canola methyl ester, a soybean methyl ester, a corn oil methyl ester, and mixtures thereof; and (c) the diphenol and sterically hindered phenol are selected, respectively, from a compound of formula (I) and a compound of formula (II):

wherein R₁ and R₃ are H or a tert-butyl group, and R₂ is H, CH₃, a tert-butyl group or —CH₂—N(CH₃)₂.
 17. The fuel composition of claim 13, wherein: (a) the fuel composition contains about 0.1% to about 0.35% by volume of a cetane improver; (b) the biodiesel comprises a fatty acid ester selected from the group consisting of rapeseed methyl ester, a canola methyl ester, a soybean methyl ester, a corn oil methyl ester, and mixtures thereof; (c) the diphenol is selected from the group consisting of tert-butylhydroquinone, 2-tert-butyl-1,4-hydroquinone, 2,5-di-tert-butyl-1,4-hydroquinone, methylhydroquinone, and 2,5-di-tert.-octylhydroquinone, and mixtures thereof; (d) the sterically hindered phenol is selected from the group consisting of 2,6-di-tert-butylphenol, 2,4-di-tert-butylphenol, 2,6-di-tert-butyl-4-methylphenol, 2-tert-butylphenol, 2,4,6-tri-tert-butylphenol, and 2,6-di-tert-butyl-4-dimethylaminomethylphenol, and mixtures thereof; (e) the fuel composition contains about 100 milligrams of antioxidant per liter of the biodiesel to about 500 milligrams of antioxidant per liter of the biodiesel; and (f) the volumetric ratio of the diphenol to the sterically hindered phenol is between about 0.5:1 to about 1.5:1.
 18. The fuel composition of claim 13, wherein: (a) the fuel composition contains about 0.1% to about 0.35% by volume of a cetane improver; (b) the biodiesel comprises a fatty acid ester selected from the group consisting of rapeseed methyl ester, a canola methyl ester, a soybean methyl ester, a corn oil methyl ester, and mixtures thereof; (c) the diphenol is selected from the group consisting of tert-butylhydroquinone, 2-tert-butyl-1,4-hydroquinone, 2,5-di-tert-butyl-1,4-hydroquinone, methylhydroquinone, and 2,5-di-tert.-octylhydroquinone, and mixtures thereof; (d) the sterically hindered phenol is selected from the group consisting of 2,6-di-tert-butylphenol, 2,4-di-tert-butylphenol, 2,6-di-tert-butyl-4-methylphenol, 2-tert-butylphenol, 2,4,6-tri-tert-butylphenol, and 2,6-di-tert-butyl-4-dimethylaminomethylphenol, and mixtures thereof; (e) the fuel composition contains about 400 milligrams of antioxidant per liter of the biodiesel; and (f) the volumetric ratio of the diphenol to the sterically hindered phenol is between about 0.75:1 to about 1:1.
 19. The fuel composition of claim 13, wherein: (a) the fuel composition contains about 0.1% to about 0.35% by volume of a cetane improver; (b) the biodiesel comprises about 50% to about 99% by weight of methyl, ethyl, and propyl esters of saturated and unsaturated fatty acids; (c) the diphenol is selected from the group consisting of tert-butylhydroquinone, 2-tert-butyl-1,4-hydroquinone, 2,5-di-tert-butyl-1,4-hydroquinone, methylhydroquinone, and 2,5-di-tert.-octylhydroquinone, and mixtures thereof; (d) the sterically hindered phenol is selected from the group consisting of 2,6-di-tert-butylphenol, 2,4-di-tert-butylphenol, 2,6-di-tert-butyl-4-methylphenol, 2-tert-butylphenol, 2,4,6-tri-tert-butylphenol, and 2,6-di-tert-butyl-4-dimethylaminomethylphenol, and mixtures thereof; (e) the fuel composition contains about 400 milligrams of antioxidant per liter of the biodiesel; and (f) the volumetric ratio of the diphenol to the sterically hindered phenol is between about 0.75:1 to about 1:1.
 20. The fuel composition of claim 13, wherein the fuel composition further comprises a composition selected from the group consisting of an aromatic amine antioxidant and a performance additive. 